1. Field of the Invention
The invention is generally related to the area of optical information recording medium. More particularly, the present is related to techniques for providing a cover layer on a Blu-ray disc, wherein the cover layer is produced with substantial even-thickness.
2. The Background of Related Art
Blu-ray, also known as Blu-ray Disc (BD) is the name of a new-generation optical disc format jointly developed by the Blu-ray Disc Association (BDA), a group of leading consumer electronics and PC companies (including Apple, Dell, Hitachi, HP, JVC, LG, Mitsubishi, Panasonic, Pioneer, Philips, Samsung, Sharp, Sony, TDK and Thomson). The format was developed to enable recording, rewriting and playback of high-definition video (HD), as well as storing large amounts of data. A single-layer Blu-ray Disc can hold 25 GB, which can be used to record over 2 hours of HDTV or more than 13 hours of standard-definition TV. There are also dual-layer versions of the discs that can hold 50 GB.
While current optical disc technologies such as DVD, DVD±R, DVD±RW, and DVD-RAM use a red laser to read and write data, the new format uses a blue-violet laser instead, hence the name Blu-ray. Despite the different type of lasers used, Blu-ray products can easily be made backwards compatible through the use of a BD/DVD/CD compatible optical pickup and allow playback of CDs and DVDs. The benefit of using a blue-violet laser (405 nm) is that it has a shorter wavelength than a red laser (650 nm), which makes it possible to focus the laser spot with even greater precision. This allows data to be packed more tightly and stored in less space, so it's possible to fit more data on the disc even though it's the same size as a CD/DVD. This together with the change of numerical aperture to 0.85 is what enables Blu-ray Discs to hold 25 GB/50 GB.
Blu-ray discs can be read at a blue-violet laser (λ=405 nm) with a numerical aperture (NA) being 0.85. However, the increase of the NA value may effectively reduce the value of the thickness of the optical path that is denoted by t, defocus is proportional to NA2/λ, the disc tilt tolerance is according to λ/(t×NA3) and the thickness variation tolerance of the optical path is λ/NA4. So it is generally concluded that the situation can be handled by controlling the error of that length and by decreasing the residual focus error. A simple calculation leads to the conclusion that around 0.1 mm thick of optical path and a few microns of error will guarantee compatible detection tolerances in the Blu-ray disc system to those in the conventional systems.
The cover layer formation is one of the characteristic processes in mass production of the Blu-ray discs. In general, a 0.1 mm-thick cover layer is placed on top of 1.1 mm-thick substrate. This appears one way that the physical specifications required by the format may be satisfied. However, the value of t is so small, the flatness requirement of the cover layer is of extremely high. Various manufacturing methods have been proposed how to generate a cover layer of 0.1 mm uniform thickness and place such a cover layer on a 1.1 mm-thick substrate.
One of the manufacturing methods to produce a cover layer is by spin coating of lacquer. Because of the inherent spinning characteristics in the manufacturing method, it is noted that it is very difficult to create a cover layer of 0.1 mm uniform thickness. To control the flow of the lacquer, an additional apparatus is provided to guide the flow of the lacquer. However, it is noted that, as the lacquer is spun outwards, the characteristics of the lacquer could cause that a certain level of jaggedness, likely the cover layer is thicker in the middle than in the edge area. In addition, the lacquer is typically cured by UV lighting. In the curing process, the lacquer in the edge are tends to shrink towards the middle. As a result, the thickness of the cover layer becomes non-uniform.
Thus there has been a great need for techniques of providing uniform cover layer of a specified thickness onto a disc.